In recent years, liquid crystal display elements of TN mode have been widely used in display systems, such as watches and table calculators. Liquid crystal materials used in TN mode display elements include nematic liquid crystals and nematic-cholesteric liquid crystals. However, these liquid crystals have a slow electro-optic response and also have difficulty in driving by a high multiplex system on an account of the fact that the driving force for moving molecules in response to an electric field is basically based on anisotropy of their dielectric constant.
In expectation of settlement of these problems, smectic liquid crystals, particularly chiral smectic liquid crystals exhibiting ferroelectric properties have recently been receiving extensive studies since Clark and Lagawall made a report in 1980 on the characteristics of these liquid crystals, which are of extreme importance for display devices, such as a high rate of response on the level of submicrosecond, a memory effect ascribed to bistability, and existence of a threshold value in voltage for writing. Because of these excellent characteristics, smectic liquid crystals and, in particular, chiral smectic liquid crystals have been attracting attention as a new material for use in display elements, and applications to large-capacity displays, memory type displays, and optical modulation elements have been attempted to date.
Performance properties required for liquid crystal materials, particularly ferroelectric liquid crystals, on practical use include (1) chemical and photochemical stability for assuring durability, (2) capability of exhibiting a liquid crystal phase in a wide temperature range from low to high and exhibiting ferroelectric properties in a temperature range including room temperature, (3) small rotation viscosity and high spontaneous polarization which make it feasible to obtain a high rate of response, (4) proper birefringence to agree with an increase in display contrast, (5) a balance of modulus of elasticity suited for dynamic driving, and (6) an appropriate angle of tilt for obtaining a high display contrast.
While various liquid crystal materials have hitherto been synthesized based on these designs, there has been developed no liquid crystal material which satisfies all the performance requirements even when used singly. Accordingly, the functions demanded for liquid crystal display systems have been fulfilled in practice by preparing a multi-component mixed liquid crystal composition comprising several to ten and several different kinds of liquid crystal materials instead of individual use thereof.
Currently available mixed liquid crystal compositions in which physical properties of a ferroelectric liquid crystal are properly adjusted are roughly classified into two types; one prepared by mixing a plurality of liquid crystal compounds showing a chiral smectic phase, and the other prepared by incorporating a liquid crystal or component showing a smectic phase (hereinafter referred to as a chiral dopant).
In the former type of compositions, it is believed relatively easy to achieve a desired extension of a temperature range wherein a chiral smectic C phase is exhibited and a desired increase in spontaneous polarization which is reflected on a rate of response to an applied electric field. However, since liquid crystal compounds showing high spontaneous polarization have such a molecular structure that the dipole is positioned close to an asymmetric carbon atom, the molecules not only have a high spontaneous polarization but also tends to have a high rotation viscosity. It follows that the resulting liquid crystal composition has difficulty in attaining a high rate of response sufficient for dynamic image display at temperatures in the vicinity of room temperature.
In the latter type of compositions, the liquid crystal or liquid crystal composition showing a smectic C phase which serves as a main component of the composition (hereinafter referred to as a mother liquid crystal) does not need to be optically active by itself and may be mixed with other components in various manners so as to satisfy the above-described requirements. Optically active compounds which, while not limited, preferably exhibit a crystal phase are added to the mother liquid crystal to realize a chiral smectic C phase and to induce appropriate spontaneous polarization to thereby provide a ferroelectric liquid crystal composition having a high rate of response.
According to the latter system, a liquid crystal composition in which the mother liquid crystal has a low viscosity can be prepared. Addition of the chiral dopant makes it feasible to control a spontaneous polarization or the spiral pitch of the chiral smectic C phase, thereby obtaining an electro-optic element having satisfactory characteristics, such as a rate of response, orientation properties, and a display contrast. For these reasons, this system is steadily extending its applicability to production of practical materials. From such a technical viewpoint, there has been a strong demand for optically active or inactive mother liquid crystals and chiral dopants which exhibit excellent performance characteristics.